Thermocouple protecting tube

ABSTRACT

The thermocouple protecting tube of this invention is characterized by having a heat-insulating layer in an annular space formed by and between an inner ceramic tube and an outer silica glass tube which are concentrically disposed within each other. Due to the above construction, the protecting tube can withstand the thermal shock which it receives when immersed in a molten body of high temperature and accordingly the thermocouple enclosed in the protecting tube can continuously measure the temperature of molten steel for a considerable length of time.

BACKGROUND OF THE INVENTION

This invention relates to a thermocouple protecting tube which canwithstand the temperature measuring operation within molten metal formany hours, thereby providing for the continuous measuring of thetemperature of molten metal with high precision.

With the increase of continuous casting facilities in steel makingplants, the continuous measuring of the temperature of molten metal hasbecome a matter of vital importance.

The purposes for continuous temperature measurement of molten steel mayvary depending on the particular steel making plant; however, ingeneral, the purposes can be summarized. as involving quality controland the lowering of production costs.

For quality control, continuous measuring is effective in the productionof metal having a uniform quality and for the prevention of segregationwithin the metal. Furthermore, continuous measuring facilitates therefining operation since the temperature of molten metal can becontinuously measured.

With a view toward lowering of production costs, continuous measuringenables the complete computarization of the control of the refiningoperation and also improves the drawing speed of the continuous castingoperation.

Conventionally, for measuring the temperature of molten steel,ceramic-made thermocouple protecting tubes, especially aluminaprotecting tubes, have been predominantly used.

In general, ceramics (sintered ceramics) used for the above purpose areeasily harmed by thermal in fact, a sharp rise or fall of temperaturecauses the rupture thereof.

However, the above ceramics have a rather high heat resistance, and theycan withstand a temperature of up to 2000° C.

This heat resistance employed hereunder implies erosion resistanceagainst molten metal, molten slag, molten glass or other chemicals ofhigh temperature, loading resistance and fluidity resistance at a hightemperature.

The basic concept of this invention involve the use of such ceramics,which have high heat resistance and low thermal shock resistance forproducing a thermocouple protecting tube, which can to be used for thecontinuous measurement of the temperature of a molten body for aconsiderable length of time under conditions of immersion.

The applicant of this invention has previously filed several patentapplications such as U.S. Pat. Application Ser. No. 715,023, now U.S.Pat. No. 4,060,095 related to thermocouple protecting tubes, wherein thefollowing provisions for protecting the ceramic thermocouple protectingtube from rupture caused by thermal shock have been disclosed, namely:

(1) coating a refractory powder onto the outer surface of the ceramictube,

(2) providing a silica glass tube concentrically the ceramic tube.

Although, the above ceramic tubes are effective when their diameter issmall, it has been found that they tend to rupture when their diameterbecomes large.

For example, when an alumina tube containing Cr₂ O₃ in an amount of 5%by weight and having an outer diameter of 5mm, an inner diameter of 5mmand a wall thickness of 2.5mm (the tube of this composition has beenspecifically developed for the purpose of this invention and accordinglyforms a part of this invention) was provided with either conventionalprovision 1 or 2 and subsequently immersed in molten steel of at about1500° C, the mean lifetime of this thermocouple protecting tube wasabout 150 to 180 minutes at maximum, due to the erosion caused by moltenslag floating on the molten steel.

To prolong the lifetime of the thermocouple and thus facilitate the morecomplete operation thereof the tube must be more resistant to erosion bymolten slag, and this can be achieved only by making the wall of thetube thicker.

However, the tube must accomodate the plutinum thermocouple elementtherein, and therefore the inner diameter of the tube must be at leastmore than 5 to 6mm.

Accordingly, the tube must have an increased outer diameter to providefor a thicker tube wall , e.g. from the conventional 10mm to 12 or 14mm.

However, as described above, it has been found that in proportion to theincrease of the outer diameter, the thermal shock resistance of the tubedecreases by the square value of the outer diameter, and thus provisions1 and 2 cannot protect tubes of increased outer diameter from rupturing.

To be more specific, the maximum outer diameters that are available forprovisions 1 and 2 are 10mm and 12mm respectively. When the outerdiameter of the tubes exceeds the above values, the tubes rupture, evenif they are preheated before they are immersed in a molten body.

In the case of provision 1, when a tube having an outer diameter greaterthan the allowable diameter was immersed in molten steel at above 1500°C, the coating layer of the tube peeled off or the temperature of thetube sharply rose due to heat transfer through the coating layer.

In the case of provision 2, when a tube of excessive diameter wasimmersed in molten steel, the silica glass tube which is disposed overthe ceramic tube was softened by the heat of the molten steel andsubsequently adhered to the ceramic tube due to the buoyancy of themolten steel, whereby the temperature of the ceramic thermocoupleprotecting tube sharply increased. That is, the air layer (low heatconducting layer) disposed between the ceramic tube and the outer silicaglass tube, which reduced the rate of heat transfer within a limitedrange, was not sufficiently effective, thereby the tube ruptured.

Furthermore, since the silica glass was transparent or semi-transparent,the glass tended to transfer the radiation heat readily.

It is believed that the temperature of the ceramic thermocoupleprotecting tube rose sharply when it was provided with either of abovetwo provisions due to the two unfavorable phenomena discussed above andaccordingly, a tube of excessive diameter could not withstand or absorbthermal shock and ruptured.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a thermocoupleprotecting tube of triplicate concentric construction which canwithstand thermal shock and can resolve the aforementioned defects ofconventional tubes used in the continuous temperature measurement ofmolten steel for a considerable time.

It is another object of the present invention to provide a thermocoupleprotecting tube which enables a thermocouple element to measure thetemperature of molten steel accurately.

It is still another object of the present invention to provide athermocouple protecting tube which can be easily manufactured.

According to the invention, the thermocouple protecting tube forcontinuously measuring the temperature of molten steel comprises:

(a) an inner ceramic tube having one end closed, and containing athermocouple element therein,

(b) an outer silica glass tube disposed concentrically over the innerceramic tube and over the one closed end of the inner tube forming anannular space therebetween, and

(c) an intermediate heat-insulating layer within the annular space.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front sectional view of the thermocouple protecting tube ofthis invention.

FIG. 2 to FIG. 5 are diagrams showing the temperature elevation curvesof the protecting tube of the first to fourth examples.

FIG. 6 is a diagram showing the measured temperature curves of aconventional tube and of the tube of this invention respectively.

FIG. 7 is a front sectional view of an inner thermocouple protectingtube made of a mixture composed of chromium oxide (1 to 35% by weight)and alumina (99 to 65% by weight).

DETAILED DESCRIPTION OF THE DISCLOSURE

As shown in FIG. 1, a silica glass tube 2 is disposed over and around aninner ceramic tube 1 made of metal oxide, metal carbide, metal nitrideor a mixture of above components, forming a desired annular space 3therebetween. In the above space 3, a heat-resistant insulating layer 4is formed. The thermocouple element 5 is disposed in the inner ceramictube 1. As for a material of the heat-resistant layer 4, the ceramicpaper of ceramic fiber which is wound around the inner ceramic tube 1,refractory powder, or cement can be considered.

As for the material of the ceramic protecting tube 1, a metal oxide(e.g. alumina, beryllia, magnesia) can be considered.

Furthermore, ceramic carbide (e.g. silica carbide), nitride carbide anda mixture of these components are also useful materials for the ceramicprotecting tube 1.

In other words, according to this invention, any ceramics which have alow thermal shock resistance can be employed as the material for thethermocouple protecting tube, which is immersed into a molten body ofhigh temperature.

The rupture of the above constructed tube can be prevented by simplyincreasing the thickness of the insulating layer 4, which can reduce therate of the temperature rise of protecting tube 1.

The ceramic paper employed herein is produced by first gatheringceramic, oriented, heat resistant fibers to form a cotton-like body andsubsequently adding a small amount of binder to the cotton-like body soas to form a paper-like material.

The thermocouple protecting tube of this invention is describedhereinafter in the light of four examples wherein all examples use thefollowing same ceramic tube 1, silica glass tube 2 and plutinum element5.

    ______________________________________                                        1)  Ceramic thermocouple protecting tube 1:                                   alumina which contains chromium oxide                                         composition:                                                                              Al.sub.2 O.sub.3                                                                      Cr.sub.2 O.sub.3                                                                      MgO   Impurities                                              % by weight:                                                                          94.44   5.0   0.05 0.01                                   These components were grained, shaped and sintered by                         conventional methods.                                                         size    outer diameter:                                                                            14mm                                                                          inner diameter:  6mm                                                          one end closed                                           2)  Silica glass tube 2:                                                      non-transparent silica glass (Toshiba ceramics S-19#)                         size    inner diameter:                                                                           19.5 0.5mm                                                                    wall thickness: 1.8  0.5mm                                 one end closed by a gas burner                                               3)  Thermocouple plutinum element 5 (PR-13%):                                 size  wire diameter:                                                                          0.5mm                                                         Insulating tube enclosing the element                                         outer diameter: 1.2mm                                                         inner diameter: 0.8mm                                                         composition of tube: alumina (Al.sub.2 O.sub.3 99.5%)                         4)  Molten steel:                                                             molten steel received in a tundish of a continuous                             casting machine                                                              ______________________________________                                    

EXAMPLE 1

Alumina powder (Showa Denko A-42) in an amount of 100 parts by weight,aluminum phosphate in an amount of 25 parts by weight and water in anamount of 30 parts by weight were mixed together within a mortar, toform a liquid mixture.

A ceramic inner tube was soaked in the above liquid mixture and then wasdried, after being removed from the mortar.

The above operations were repeated until a layer 4 of about 2mm compoundof the mixture of alumina and aluminum phosphate was formed uniformlyaround the ceramic inner tube 1.

Subsequently, the thus coated inner tube 1 was left in a room for aboutthree days to dry.

Then a silica glass tube 3 was concentrically disposed over the aboveprepared inner ceramic tube forming a thermocouple protecting tube ofthis invention as shown in FIG. 1.

The protecting tube was constructed so that the insulating layer 4(alumina powder coating layer) was disposed concentrically over theceramic protecting tube, and furthermore the silica glass outer tube 2was disposed over the above insulating layer 4.

After constructing the protecting tube in the above manner, a plutinumelement 5 was inserted into the tube.

The thus prepared tube was directly immersed in molten steel of about1520° C.

In FIG. 2, the temperature elevation curve of this protecting tube isshown as curve B, while the temperature elevation curve of a protectingtube, which is constructed by merely disposing the silica glass tubeover the ceramic protecting tube, is shown as curve A. In other words,the latter tube was not provided with an intermediate layer 4.

Upon comparing curve A and curve B, curve A of the tube which has nointermediate layer 4 shows an acute elevation in the temperature of thetube (namely, the temperature reached 1200° C., 60 seconds after it wasimmersed in molten steel); while the tube of curve B, which is providedwith the alumina-powder coating 4 between the ceramic protecting tube 1and the silica glass tube 2 does not shown the above acute elevation,but a gradual elevation of the temperature of the tube. Namely, withrespect to the latter tube, the speed of temperature elevation speed wasreduced by the provision of the intermediate layer 4.

The tube of curve A showed an excessively fast temperature elevationwith the result that cracks occurred within the inner ceramic tube.Accordingly, the temperature measuring operation by this tube wasinterrupted as soon as the silica glass tube melted away and thelifetime of the tube was only 15 minutes.

On the other hand, the tube of curve B could continue an accuratetemperature measuring operation for more than six and a half (6 1/2)hours without being eroded by molten slag after the tube was immersed inmolten steel.

Accordingly, protecting tubes of large diameter and large thicknesshaving conventional construction which have ruptured can now withstandthermal shock for a long time and thus a long and accurate temperaturemeasuring operation can be carried out.

EXAMPLE 2

A ceramic paper of the following particulars was wound twice around theceramic tube 1 including the closed end thereof and this paper-coatedceramic tube was concentrically disposed within a silica glass tube 4.The thus prepared protecting tube was immersed in molten steel at about1520° C.

    ______________________________________                                        Particulars of ceramic paper -                                                Manufacturer: Japan Asbestos Co., Ltd.                                        Trademark : Finelex 1500                                                      Composition: Al.sub.2 O.sub.3                                                                       SiO.sub.2                                                                              Na.sub.2 O                                                                           Fe.sub.2 O.sub.3                         % by weight:                                                                              64.12    35.68    0.04   0.02                                    The fibers of 3.6μ, each of which has the above                             composition were bound with an organic binder and the                         thus bound material was manufactured into papers by                           a paper making device.                                                       ______________________________________                                    

In FIG. 3, four curves A, A', C and C' are described wherein curves Aand A' are those of protecting tubes which are made by disposing onlythe silica glass tube over the ceramic tube.

Curve A corresponds to the curve A of FIG. 2 and is shown for referencepurpose.

Curve A' and C' were obtained by carrying out the example in such awaythat the tubes were preheated within the atmosphere of the tundish untilthe temperature of the tubes reached 1000° C. As soon as the abovetemperature was obtained, the tubes were immersed in molten steel.

Curves A and C are temperature elevation curves derived by directlyimmersing the tubes in molten steel.

Upon reviewing the curves A and C (direct immersing), curve A reached1200° C, 60 seconds after the immersing operation as previouslymentioned, whereas curve C of the protecting tube which had been woundtwice with ceramic paper (4mm in total thickness) reached 670° C, 60seconds after the immersing operation. Thus curve C showed a slowertemperature elevation than curve A.

Accordingly, the tube of curve A could be used to carry out a continuousmeasuring operation for only about 15 minutes, whereas the tube of curveC, which was provided with the ceramic paper made intermediate layer 4,could be used to carry out a continuous measuring operation for morethan 7 hours.

Upon reviewing curves A' and C' (both being preheated up to 1000° C),curve A' reached 1000° C about 180 seconds after the tube was disposedinto the tundish atmosphere, whereas curve C' reached 1000° C about 370seconds after the tube was disposed within the above atmosphere.

In this way, the effect of the ceramic paper intermediate layer 4 alsocan be judged by the differences of the above temperature curves even atthe preheating stage of the temperature measuring operation.

Furthermore, FIG. 3 also shows that curve A' took about 80 seconds toprovide the temperature elevation of 1000° C to 1500° C whereas curve C'necessitated more than 240 seconds to reach the above temperatureincrease.

Notwithstanding that the tube of curve A' was preheated to 1000° C, thetube could withstand the continuous measuring operation for only 12minutes.

This implies that the temperature of the above tube sharply increasedeven when the tube was immersed in molten steel after the preheatingoperation, so that cracks occurred in ceramic inner tube 1 resulting inthe short temperature measuring operation.

On the other hand, the tube provided with the ceramic paper 4, whichworks as an insulating material, could continue measuring thetemperature of molten steel for more than 7 hours.

Furthermore, it is considered that the tube which may be wound by acloth made of ceramic fiber or glass fiber has the same effect as thatof the previously discussed ceramic-paper-wound tube.

EXAMPLE 3

This example relates to a protecting tube which is constructed bycoating an alumina cement on a ceramic inner tube and subsequentlyconcentrically enclosing the alumina-coated inner tube with a silicaglass outer tube.

The coating of the alumina cement was carried out as follows.

(1) Alumina cement in an amount of 100 parts by weight and water in anamount of 30 parts by weight were mixed together.

(2) A cylinder having an inner diameter sufficient to provide a cementcoating of 2mm thickness on the surface of the ceramic tube was formedby a polyester plate.

(3) The cylinder was placed on the clay.

(4) The ceramic tube was concentrically inserted into the cylinder untilthe lower closed end thereof was slightly above the clay surface.

(5) While maintaining the ceramic tube in the above position, the liquidcement was filled in the space between the inner ceramic tube and outercylinder so that the liquified cement covered the bottom round portionof the ceramic tube.

(6) Next day, the cylinder was removed from the inner tube.

(7) Subsequently, the silica glass tube concentrically enclosed thealumina cement coated inner tube.

The thermocouple protecting tube which was made in the above manner wasimmersed in molten steel of about 1520° C, wherein FIG. 4 shows thetemperature elevation curve D of the above tube and that of theconventional curve A.

Upon reviewing the above curve A and the curve D, the temperature of thecurve A reached 1200° C, 60 seconds after the tube was immersed inmolten steel, whereas the temperature of the curve D reached only 600°C, 60 seconds after immersing the tube. Therefore, the latter tube had aslower temperature elevation than the former one. Accordingly, even whenthe tube was subjected to thermal shock, no cracks occurred in the innerceramic tube and the tube could carry out a continuous temperaturemeasurement for about 6 hours, whereas the tube of curve A could measurethe temperature for only 15 minutes.

Therefore, it is safe to say that the tube constructed by enclosing acement coated ceramic tube in a silica glass tube is highly effectivefor measuring the temperature of molten steel, since the cement works asan insulating material to absorb the thermal shock which tends torupture the ceramic protecting tube.

EXAMPLE 4

This example relates to a thermocouple protecting tube which isconstructed by filing a desired amount of alumina powder in the spaceformed by and between an inner ceramic tube and an outer silica glasstube, which tubes are arranged concentrically. FIG. 5 shows thetemperature elevation curve E of this tube and curve A of a conventionaltube.

Upon reviewing these two curves, the temperature of curve A reached1200° C, 60 seconds after the tube was immersed in molten steel, whileit took 106 seconds to reach 1500° C. Whereas the temperature of curve Ereached a mere 250° C in the original immersing and it took about 420seconds to reach 1500° C.

Accordingly, no cracks occured in the tube of curve E when the tube wasimmersed in molten steel, so that the tube could carry out thecontinuous measurement of temperature for more than 7 hours. As had beendescribed heretofore, the tube of curve A could measure the temperaturefor only 15 minutes.

Therefore, it is believe that the thermal shock that the ceramicprotecting tube received was absorbed by the alumina powder filled inthe space formed between inner and outer tubes.

Furthermore, to investigate the accuracy of the temperature measurementand the retarding effect of the protecting tube of the first example,the temperature measured by the protecting tube of this invention andthe temperature measured by the conventional tube were graphicallycompared in FIG. 6.

The conventional tubes could carryout only instantaneous temperaturemeasurements and accordingly the measured temperatures were designatedby "X. " These designated temperatures were connected by a dotted linefor convenience in evaluating the comparison chart hereunder. It is alsoto be noted that the periodical reduction of measured temperature in thechart (FIG. 6) were the temperatures measured when the tundish was aboutto be tapped with fresh molten steel from a ladle.

As can be clearly observed from FIG. 6, the thermocouple protecting tubeof this invention can carry out accurate temperature measurements for along time, provided that the thickness of the tube, which is 4mm beforethe measuring operation, is reduced to 2.8mm in the final stage of themeasuring operation.

As has been described heretofore, due to the provision of theintermediate layer which is placed between two concentric tubes, theinner ceramic tube and the outer silica glass tube, the temperatureelevation speed of the thermocouple protecting tube can be reduced so asto be as slow as possible. Thus, a tube of large diameter, whichconventionally ruptured easily when it received a thermal shock can nowbe used for temperature measuring purposes.

Of course, it is needless to say that a ceramic, which is of high heatresistance but of low thermal shock resistance, can be used as thematerial for thermocouple protecting tubes.

It is also comtemplated that the temperature elevation speed can bechanged at will by merely changing the thickness or the material of theinsulating layer.

As previously discussed, an alumina ceramic tube (containing Cr₂ O₃ inan amount of 5% by weight) which has been either coated with refractorypowder or enclosed by a silica glass tube can be used as thethermocouple protecting tube to be immersed in molten steel, providedthat the diameter and the thickness thereof is a maximum of 10mm and12mm respectively. Therefore, this protecting tube can conduct acontinuous measuring operation of molten steel only for 150 to 180minutes after being immersed in the tundish.

Whereas with the provision of this invention, the diameter and thethickness of the protecting tube can be increased to 14mm and 4mmrespectively, whereby the lifetime of the protecting tube forcontinuously measuring the temperature of molten steel is greatlyprolonged, namely more than 7 hours. This implies that a singleprotecting tube is sufficient to measure the temperature change ofmolten steel during the whole steel making operation.

Accordingly, the thermocouple protecting tube of this invention has thefollowing advantages.

(1) Since the continuous temperature measuring operation can be effectedby a single protecting tube, the cost for the measuring operation can begreatly reduced.

Normally when a protecting tube of conventional construction is used forthe temperature measuring operation at least 3 protecting tubes arerequired for continuous measuring over a period of seven hours, whereasa single protecting tube of this invention is sufficient to cover thisoperation. In addition, the dangerous operations of immersing andremoving the tube into and from a molten body can be reduced.

(2) Since the protecting tube of this invention has a large diameter (orlarge thickness), the lifetime of the tube can be prolonged.

Thus, the above protecting tube can protect the ceramic tube fromrupturing.

(3) Due to the provision of the insulation layer which is formed byinserting the insulating material between the inner ceramic tube and theouter silica glass, the erosion of the tube, which occurs due to moltenslag, can be prevented resulting in a prolonged lifetime of theprotecting tube.

(4) The rupture of the thermocouple protecting tube by thermaldeflection, which occurs when there is a temperature difference betweenthe soaked portion and unsoaked portion of the tube during the soakingoperation, can be eliminated.

In the previous examples, we have used ceramic composed of chromiumoxide (5.0% by weight) and alumina (94.44% by weight) as a preferredmaterial for producing the inner tube of the thermocouple protectingtube, which is constructed either conventionally or in accordance withthis invention. However, we have found as shown by the following examplethat inner tubes containing chromium oxide in an amount of 1 to 35% byweight also perform well in carrying out the temperature measurement ofmolten steel.

EXAMPLE 5

We have prepared the thermocouple protecting tubes constructed as shownin FIG. 7 (all of them have the same outer diameter of 7mm, and the samewall thickness of 1.5mm), which are made by mixing different amounts ofchromium oxide with alumina respectively as set forth below.

Then they were immersed into molten steel at about 1600° C, on whichslag thereof has been floated, and were used for the continuousmeasuring of temperature.

It must be noted that the above (inner) tubes were provided with neitherouter tubes nor an intermediate insulating layer and that the abovetubes had an outer diameter (7mm) smaller than that (14mm) of theforegoing Examples 1 to 4.

The results of the above Example are shown in the following chart:

    ______________________________________                                        Cr.sub.2 O.sub.3 (% by weight)                                                                Lifetime (minutes)                                            ______________________________________                                         0              20                                                             1              40                                                             5              53                                                            10              55                                                            15              60                                                            20              60                                                            25              55                                                            30              53                                                            35              52                                                            40              Ruptured by thermal shock                                     45              Ruptured by thermal shock                                     ______________________________________                                    

In the above Example, the temperature of the thermocouple element becameequal to that of the molten steel about 40 seconds after thethermocouple protecting tube was dipped into the molten steel.

From the above chart, it has been found that:

(1) when the amount of Cr₂ O₃ is less than 1% by weight, the lifetime ofthe tube is short,

(2) when the above CR₂ O₃ amount is more than 35% by weight, the tuberuptures.

Therefore, it can be understood that the amount of Cr₂ O₃ should be from1 to 35% by weight and preferably from 15 to 20% by weight. To be morespecific, the optimal amount of Cr₂ O₃ will be from 15 to 20% by weight.

It was also believed that although tubes containing Cr₂ O₃ of 40% byweight and 45% by weight were sintered at 1900° C, this temperature wastoo low to sinter the tubes sufficiently.

Since the tubes had a high percentage of Cr₂ O₃, their refractorinesswas increased, so that the above sintering temperature (1900° C) was toolow to impart sufficient strength to the tubes to withstand thetemperature of molten steel.

Therefore, it will be safe to say that if the sintering temperature ofabove tubes could be raised, the tubes of high Cr₂ O₃ content (e.g. morethan 40% by weight) could withstand the temperature of molten steel.

It may be possible to add MgO in addition to Cr₂ O₃ as an agent forrestraining the growth of crystal particles of Cr₂ O₃. It is alsopossible to add CaO or SiO₂ in addition to MgO and Cr₂ O₃, whereby thesinterability of the tube can be improved.

Additionally, another Example relating to the Example 5 has shown thattubes having a diameter of more than 10mm frequently rupture due tothermal shock when they are subject to a molten body of about 1600° C.

As has been described, the thermocouple protecting tube of thisinvention can remarkably prolong a lifetime of the thermocouple, wherebycontinuous measurement of the temperature of a molten body can beachieved.

What we calim is:
 1. A thermocouple protecting tube for continuouslymeasuring the temperature of molten steel comprising:(a) an innerceramic tube having one end closed, and containing a thermocoupleelement therein, (b) an outer silica glass tube disposed concentricallyover said inner ceramic tube and over said one closed end of said innertube forming an annular space therebetween, and (c) an intermediateheat-insulating layer within said annular space.
 2. A thermocoupleprotecting tube according to claim 1, wherein said inner ceramic tube ismade of metal oxide selected from the group consisting of alumina,beryllia and magnesia.
 3. A thermocouple protecting tube according toclaim 1, wherein said inner ceramic tube is made of metal carbide.
 4. Athermocouple protecting tube according to claim 1, wherein said innerceramic tube is made of ceramic carbide.
 5. A thermocouple protectingtube according to claim 1, wherein said inner ceramic tube is made ofmetal nitride.
 6. A thermocouple protecting tube according to claim 1,wherein said inner ceramic tube is made of a mixture of metal oxide,metal carbide and ceramic carbide.
 7. A thermocouple protecting tubeaccording to claim 6, wherein said misture is composed of chromium oxidein an amount of about 1 to 35% by weight and alumina in an amount ofabout 99 to 65% by weight.
 8. A thermocouple protecting tube accordingto claim 1, wherein said intermediate heat-insulating layer is comprisedof ceramic paper wound around said inner ceramic tube.
 9. A thermocoupleprotecting tube according to claim 1, wherein said intermediateheat-insulating layer is comprised of ceramic fiber wound around saidinner ceramic tube.
 10. A thermocouple protecting tube according toclaim 1, wherein said intermediate heat-insulating layer is comprised ofalumina cement of uniform thickness coated onto the outer surface ofsaid inner ceramic tube.
 11. A thermocouple protecting tube according toclaim 1, wherein said intermediate heat-insulating layer is comprised ofalumina powder filled in said annular space.
 12. A thermocoupleprotective tube according to claim 1, wherein said intermediate heatinsulating layer covers the outside of said inner ceramic tube includingthe closed end thereof but does not completely fill the space betweenthe closed end of said inner ceramic tube and said outer glass tube.